1. Field of the Invention
The present invention relates to a method of producing a molded ceramic product, the molded ceramic product produced by this method, and an image-taking apparatus mounted with the molded ceramic product.
2. Description of the Related Art
In recent years, automobiles have been provided with cameras. Meanwhile, nowadays, automobiles are equipped with display screens thanks to the widespread use of car navigation systems. Therefore, many of cameras installed in automobiles can display the state of a blind spot for a driver on a display screen provided in front of the driver's seat.
Incidentally, automobiles may be left outdoors in hot summer and in freezing winter and therefore, a camera installed in an automobile is required to operate properly over an extremely wide range of temperatures. In order to guarantee the proper operation of a vehicle-mounted camera, a lens assembly employed in such a camera needs to be not only lightweight and robust as generally required, but also resistant to stretching and shrinking with temperature for the purpose of preventing the occurrence of a blur due to a change in temperature. To meet these needs, ceramic barrels have been devised (see, for example, Japanese Patent Application Publications No. 2006-284991 and No. 2006-292927). Also, the inventors of the present application have proposed to apply porous ceramic to an optical component (see, for example, Japanese Patent Application Publication No. 2007-238430). The porosity of the porous body described in the specification of Japanese Patent Application Publication No. 2007-238430 is equal to or more than 10%.
Further, for instance, Japanese Patent Application Publication No. 2007-279557 describes a lens assembly having such a structure that lenses and spacing rings are inserted into a lens barrel through the front opening of the lens barrel, and a pressing ring is threaded on a front part of the lens barrel, thereby fixing the lenses to the lens barrel. A structure in which a ceramic barrel is applied to the structure of Japanese Patent Application Publication No. 2007-279557 will be described.
FIG. 1 is a diagram that illustrates an example of the structure of a lens assembly provided with a ceramic barrel.
A lens assembly 1 illustrated in FIG. 1 is provided with a barrel 10, and the barrel 10 has a hollow part 100 having an object-side opening 101 and an image-forming-side opening 102. Formed on an object-side periphery of the barrel 10 is a male thread SR1. From the object-side opening 101, lenses L1 trough L4 and spacing rings SP1 through SP3 are inserted while being aligned along the optical axis. These elements L1 through L4 and SP1 through SP3 may be collectively referred to as optical members. In this example, the lenses L1 trough L4 and the spacing rings SP1 through SP3 are alternately disposed and sequentially inserted into the hollow part 100 of the barrel 10.
Furthermore, the lens assembly 1 illustrated in FIG. 1 is provided with a pressing ring 11 that fixes the optical members L1 through L4 and SP1 through SP3 inserted into the hollow part 100 of the barrel 10, by pressing these optical members from the side where the object-side opening 101 is formed. The pressing ring 11 has: a mounting opening 110 into which an object-side part of the barrel 10 is inserted; and an optical opening 111 for making a central part of the lens L1 exposed. Among the optical members inserted into the barrel, the lens L1 is the one disposed at the position closest to the object. Formed on an inner wall on a mounting opening side of the pressing ring 11 is a female thread SR2 in which the male thread SR1 is engaged. When the male thread SR1 is engaged in the female thread SR2, the edge of an object-side surface of the lens L1 disposed at the position closest to the object on the object side is pressed by the pressing ring 11.
The lens assembly 1 shown in FIG. 1 is assembled by using the pressing ring 11 that presses the lenses L1 trough L4 and the spacing rings SP1 through SP3 in the barrel 10 toward the image-forming-side opening.
As mentioned above, the lens assembly may be installed in an automobile. Considering this fact, it is conceivable to employ a molded ceramic product as the barrel 10 of the lens assembly 1 illustrated in FIG. 1. When a molded ceramic product is employed as the barrel 10 of the lens assembly 1 illustrated in FIG. 1, a thread must be formed on the molded ceramic product by molding so that the molded ceramic product can be threaded into the pressing ring 11.
In this type of ceramic component, a thread part may be formed by cutting or the like, which requires processing time and cost. In contrast, when a thread part is formed by injection molding, processing for forming the thread part is eliminated, which is advantageous in view of processing time and cost. Nevertheless, if the molded product obtained by the injection molding is fired, there occurs such a problem that the crest of a thread is rounded off because a firing reaction takes place after mixed resin comes out of the molded product.
FIG. 2 is a diagram showing the structures of the thread SR1 of the barrel 10 and the thread SR2 of the pressing ring 11.
As mentioned above, when the molded product is fired after the thread part is formed by the injection molding, a firing reaction occurs after the resin comes out of the molded product. Therefore, the crest and trough of the thread are inevitably rounded as illustrated in part (a) of FIG. 2. As illustrated in part (b) of FIG. 2, when the bottom of the trough of the barrel 10 made of ceramic is rounded, the corresponding crest of the pressing ring 11 made of, for example, aluminum cannot be engaged in this trough of the barrel 10.